Lignin is a class of complex organic polymers that form key structural materials in the support tissues of certain plants. It is fundamental in the formation of cell walls in wood and bark. It is also a by-product of biorefinery processes, which researchers have found a great, new use for. Now, Oak Ridge National Laboratory has developed lignin composites that could serve as renewable 3D printing feedstock. The materials gained many characteristics desirable for extrusion and producing objects.
Lignin is one of the leftovers from the production of bioproducts and processing of biomass. Lignin’s role in plant biology is in improving rigidity of cellular structures, so its utility in 3D printing is clear. The things that let it give plants structure and also it’s nature as an insoluble remnant of the plant process make make it easy to mix with plastics to derive filaments and pellets.
“Finding new uses for lignin can improve the economics of the entire biorefining process,” said ORNL project lead Amit Naskar. In fact, the fundamental goal of the research is to make better use of bioproducts and bioprocesses. The research was, after all, funded by DOE’s Office of Energy Efficiency and Renewable Energy’s Bioenergy Technologies Office.
Manufacturing Renewable Composites
The researchers created lignin composites by melting the material with conventional plastics. They used melt-stable hardwood lignin with a low-melting nylon and carbon fiber to give the resulting material good viscosity and weld strength for layer on layer deposition and mechanical properties.
While plain lignin can heat up and char (unlike the standard 3D printing thermoplastics) the composite fared better in the additive manufacturing process. Lignin also undergoes an increase in viscosity as it heats up, making it difficult to extrude. This posed as a point of reservation for the researchers, but much to their surprise, the composite’s room temperature stiffness increased while its melt viscosity decreased upon mixing with Nylon.
They also found that the mixture of materials created a lubricating effect. The researchers studied these new characteristics with neutron scattering at the High Flux Isotope Reactor and advanced microscopy at the Center for Nanophase Materials Science. The researchers experimented with varying mixtures of lignin, testing up to 40 to 50 percent by weight. This was a breakthrough in this field of research, being the highest by far. They then added 4 to 16 percent carbon fiber into the mix as well, creating a composite that heats up more easily, flows faster for speedier printing, and results in a stronger product.
Currently, their work with the Lignin-Nylon composite is patent-pending. ORNL are still refining the process and finding other ways to process the mixtures. It could also become a roadmap to future materials research with other natural composites and waste products.
Featured images courtesy of Oak Ridge National Laboratory, retrieved via their media.